The present disclosure relates generally to the field of underwater geopositioning systems and methods.
Undersea mobile or autonomous systems do not have access to positioning assets, such as global positioning system (GPS) or radio frequency (RF) assets, that are available to other non-submersible vehicles and systems. The need for navigational assistance beneath the water is further exacerbated by the general lack of available visible references in such undersea systems.
One technique that may be used to provide positioning data for undersea vehicles is a long baseline (LBL) method that operates by relying on a series of fixed underwater transponder beacons. A transducer on the mobile system emits a signal that the beacons detect, after which the beacons emit response signals. The mobile estimates its distance from each of the beacons by timing the travel of the signals, thus enabling it to calculate its own position relative to the known positions of the beacons.
LBL systems require extensive preparation and surface expression by the deploying asset (e.g., a small craft)—a factor of importance in military applications. LBL systems employ an assumption of sound being spherically radiated from multiple distant source nodes (using either clocks or transponder approaches). At the mobile, one employs an algorithm that relates the intersection of spheres to a common point. This point can only be calculated if the mobile has a priori knowledge of the positions of the multiple sound sources. If the positions are known in a 3-dimensional Cartesian coordinate system, then the mobile locates itself within that system. If the geo-locations of the sources are also known, then the mobile can also position itself within global coordinates. The locations of the sources must be pre-programmed into the mobile prior to release of the unit. Accordingly, LBL systems require that the source nodes be surveyed after deployment to determine their position before they can be used to locate the mobile system. When conducted in deep water, the survey may take days. Further, the survey uses pings from a surface ship that can be substantially affected by acoustic conditions present, which in turn can affect the accuracy of the source node position measurements. LBL systems may also require that a sound velocity profile be measured in order to correct the range measurements, which can require additional time and equipment.